This invention relates to the rechargeable electrical energy cells, such as nickel-cadmium cells, and more specifically to an improved venting valve arrangement for relieving excess gas pressure within such cells.
Under standard operating conditions, electrical energy cells such as nickel-cadmium cells, operate with internal pressure under an equilibrium condition and the container construction is designed to withstand the normally expected pressures which are generally several times that of atmospheric pressure. However, under certain conditions such as too-rapid charging, internal pressures may rise high enough to rupture the container. At one time, a rupturable diaphragm or membrane was utilized to prevent possible explosion of the container. Since, however, rupture of the sealing diaphragm also destroyed the usefulness of the cell, better devices for venting were devised which would relieve excess pressure, then reseal the interior. In most energy cells the negative terminal is provided in the form of a cup-shaped container or casing with the positive terminal being a central contact which is combined with an insulating element to form the top closure of the container. In prior structures, the positive terminal is generally comprised of a flat metal cover plate with a central aperture and a metal cap over the aperture with some form of molded resilient seal retained between the cover plate and the cap. These seals have taken many forms, some being elaborate constructions including metal spring structures combined with bonded resilient coatings. Other, less complex structures have embodied spherical configuration, for example, requiring selective positioning within the dimple in the vent cap, and still others being in the form of flattened discs having a thickened rim and requiring a seating ring around the aperture in the cover plate for positioning. The latter described disc arrangement has the advantage of essentially filling the cap and needs no positioning device but, because the leakage path is longer, requires a higher internal pressure before relief is obtained. All of the prior devices, however, have the disadvantage of being relatively expensive to produce and, in the case of the molded disc, because of the difficulty of curing small molded parts so as to achieve identical physical characteristics, even units from a single production lot would not relieve at exactly the same desired pressure.